This section is intended to introduce various aspects of the art, which may be associated with exemplary embodiments of the present techniques. This discussion is believed to assist in providing a framework to facilitate a better understanding of particular aspects of the present techniques. Accordingly, it should be understood that this section should be read in this light, and not necessarily as admissions of prior art.
Modern society is greatly dependant on the use of hydrocarbons for fuels and chemical feedstocks. However, easily harvested sources of hydrocarbon are dwindling, leaving less accessible sources to satisfy future energy needs. As the costs of hydrocarbons increase, these less accessible sources become more economically attractive. For example, the harvesting of oil sands to remove hydrocarbons has become more extensive as it has become more economical. The hydrocarbons harvested from these reservoirs may have relatively high viscosities, for example, ranging from 8 API, or lower, up to 20 API, or higher. Accordingly, the hydrocarbons may include heavy oils, bitumen, or other carbonaceous materials, collectively referred to herein as “heavy oil,” which are difficult to recover using standard techniques.
Several methods have been developed to remove hydrocarbons from oil sands. For example, strip or surface mining may be performed to access the oil sands, which can then be treated with hot water or steam to extract the oil. However, deeper formations may not be accessible using a strip mining approach. For these formations, a well can be drilled to the reservoir and steam, hot air, solvents, or combinations thereof, can be injected to release the hydrocarbons. The released hydrocarbons may then be collected by the injection well or by other wells and brought to the surface.
A number of techniques have been developed for harvesting heavy oil from subsurface formations using thermal recovery techniques. Thermal recovery operations are used around the world to recover liquid hydrocarbons from both sandstone and carbonate reservoirs. These operations include a suite of in-situ recovery techniques that may be based on steam injection, solvent injection, or both. These techniques may include cyclic steam stimulation (CSS), steamflooding, and steam assisted gravity drainage (SAGD), as well as their corresponding solvent based techniques.
For example, CSS techniques include a number of enhanced recovery methods for harvesting heavy oil from formations that use steam heat to lower the viscosity of the heavy oil. The CSS process may raise the steam injection pressure above the formation fracturing pressure to create fractures within the formation and enhance the surface area access of the steam to the heavy oil, although CSS may also be practiced at pressures that do not fracture the formation. The steam raises the temperature of the heavy oil during a heat soak phase, lowering the viscosity of the heavy oil. The injection well may then be used to produce heavy oil from the formation. The cycle is often repeated until the cost of injecting steam becomes uneconomical, for instance if the cost is higher than the money made from producing the heavy oil. However, the steam in successive steam injection cycles reenters earlier created fractures and, thus, the process becomes less efficient over time. CSS is practiced using both vertical and horizontal wells.
Solvents may be used in combination with steam in CSS processes, such as in mixtures with the steam or in alternate injections between steam injections. The solvents are typically liquid hydrocarbons at surface conditions that may be directly mixed and flashed into the injected steam lines or injected into the CSS wellbores and further transported as vapours to contact heavy oil surrounding steamed areas between adjacent wells. The injected hydrocarbons may be produced as a solution in the heavy oil phase. The loading of the liquid hydrocarbons injected with the steam can be chosen based on pressure drawdown and fluid removal from the reservoir using lift equipment in place for the CSS.
As a field ages, the use of CSS may gradually be replaced with non-cyclic techniques, for example, in which steam is continuously injected into a first well, and fluids are continuously produced from a second well. These techniques may generally be termed steamflooding, and are generally based on vertical wells. However, the use of horizontal wells is becoming more common. Steam and any other vaporized injected fluids have a tendency to override the hydrocarbons in the formation, and directly travel from injector to producer, potentially lowering their effectiveness in recovering the oil.
Another group of techniques is based on a continuous injection of steam through a first well to lower the viscosity of heavy oils and a continuous production of the heavy oil from a lower-lying second well. Such techniques may be termed “steam assisted gravity drainage” or SAGD.
In SAGD, two horizontal wells are completed into the reservoir. The two wells are first drilled vertically to different depths within the reservoir. Thereafter, using directional drilling technology, the two wells are extended in the horizontal direction that result in two horizontal wells, vertically spaced from, but otherwise vertically aligned with the other. Ideally, the production well is located above the base of the reservoir but as close as practical to the bottom of the reservoir, and the injection well is located vertically 3 to 10 metres (10 to 30 feet) above the horizontal well used for production.
The upper horizontal well is utilized as an injection well and is supplied with steam from the surface. The steam rises from the injection well, permeating the reservoir to form a vapour chamber that grows over time towards the top of the reservoir, thereby increasing the temperature within the reservoir. The steam, and its condensate, raise the temperature of the reservoir and consequently reduce the viscosity of the heavy oil in the reservoir. The heavy oil and condensed steam will then drain downward through the reservoir under the action of gravity and may flow into the lower production well, whereby these liquids can be pumped to the surface. At the surface, the liquids flow into processing facilities where the condensed steam and heavy oil are separated, and the heavy oil may be diluted with appropriate light hydrocarbons for transport by pipeline.
However, the techniques discussed above may have difficulty with removing fluids from the well bore. Artificial lifting techniques can be used to boost the amount of fluids removed from reservoirs. Such techniques include, for example, pumps, gas lift, and the like. Pumps can include surface driven pumps, such as pump jacks and the like. However, pumpjacks may not be efficient for heavy oil recovery, due to variations in flow rates, pressures, and material viscosities. Pump jacks may also have limited volumetric capacity. Down hole electrical pumps can be more effective, but may not operate well at the higher temperatures present during a high temperature recovery process, such as a steam assisted hydrocarbon production. Gas lift systems may provide a method for harvesting fluids, but require large amounts of high pressure gas be driven into the well and the associated infrastructure to supply the gas. The compression and recovery of the gas may add a significant cost to the field. In some cases natural lift is sufficient for most of the operating period and supplemental lift so an inexpensive supplemental lift system is all that is required. Thus, research has continued in techniques for lifting fluids from reservoirs.
U.S. Pat. No. 4,397,612 to Kalina, et al., discloses a gas lift system utilizing a liquefiable gas that is introduced into a well. The method includes introducing a liquid into a first well conduit to maintain a liquid level and provide a significant liquid column pressure at the downhole region of the well. The fluid passes into a second well conduit to mix with well fluid in the second conduit and cause lifting of the well fluid in the second well conduit.
In the system described above, the lifting occurs as pressure is relieved on the liquid, allowing the liquid to flash and form gas bubbles, which drive the fluids to the surface. However, the flashing of the fluids removes energy from the environment and, thus, sufficient thermal energy must be present for the flashing to occur. Further, the liquid is prevented from flashing in the first conduit by the liquid level.